The present invention generally relates to monitoring, diagnostics and predictive maintenance of an electric power generator, and more particularly, to short-circuit protection of the generator.
A generator is equipped with protective relays that constantly monitor a status of the generator, a prime mover of the generator, and of an electric power system providing power to the generator. The protective relays respond automatically to abnormal conditions that could jeopardize the generator, the prime mover, or alternatively the power system.
A fault, such as a short-circuit, internal to the generator is an important subset of the abnormal generator conditions. If left in place, the short-circuit leads to a rapidly escalating damage of the generator in terms of thermal, mechanical and electrical destruction or degradation of a plurality of windings of the generator, an isolation of the generator, and an iron core of the generator. The longer the fault, the more extensive a damage to the generator, the longer the subsequent repair time and higher a repair cost. The damage may quadruple when the fault time is doubled, leading to a point of damage beyond repairs. Preventing the damage or reducing further damage to the generator calls on a generator protection system to decouple the power system from the generator as fast as possible in case of the fault. An expected reaction time is a range of milliseconds for high current faults, to seconds for less intrusive faults such as a first stator ground fault.
Operating principles of the protective relays follow general industry standards with proprietary enhancements aimed at achieving better performance, such as, faster and more sensitive detection of the abnormal conditions, corrective actions to mitigate an impact on the power system, and to limit the damage to the generator. Normally, a corrective action is to open a circuit breaker to isolate the power system from the generator. Opening the circuit breaker connecting the generator to the power system removes an external source of energy that feeds the fault. This traditional protection action including tripping is not sufficient in the case of the generator, as the generator itself is a source of energy. As a result, other actions may be initiated in order to accelerate the shut down of the power generation process. In any case, the generator is tripped offline and a number of supporting processes related to converting fuel into electricity are interrupted in an emergency mode.
False-positives, which include operating of the protective relays when not necessary, generate significant costs associated with a shut down of the generator. Failure to operate, which is a lack or alternatively a delayed operation of the protective relays, generates losses associated with greater damage to the generator and other equipment. The failure to operate also results in longer repair times, lost revenue during the repair time, or lost equipment if the damage is beyond repair.
Failure modes of the generator include a stator ground fault and an inter-turn fault in split-phase windings of the generator. Both these failures are difficult to detect by the protective relays. The stator ground fault is a short-circuit between any of a plurality of windings of a stator of the generator and ground, via the iron core of the stator. The generator may be interfaced with the power system via a delta-connected step-up transformer and works with a neutral point of the generator ungrounded or alternatively grounded via a relatively large resistance. As a result, an amount of a short-circuit current during a stator ground fault is driven by an amount of capacitive coupling in the generator and the step-up transformer. Therefore, when the stator ground fault occurs, very small capacitive current flows making the short-circuit difficult to detect. No or very little damage is done to the generator as a result of a first stator ground fault. If however, with the first stator ground fault in place, a second stator ground fault happens, high currents start to flow, leading to quickly escalating event and potentially catastrophic damage to the generator.
The generator may be built with multiple parallel windings in each phase of the stator of the generator. The parallel windings, by the nature of their internal arrangement, are more prone to the inter-turn fault. Due to effectively very high turn-ratio between the parallel windings and a plurality of shorted coils, the inter-turn fault causes extremely high currents in a faulted loop leading to a quickly progressing damage. At the same time, there are none, or very little symptoms of the inter-turn fault measurable in a plurality of signals outside of the generator. Typically, the inter-turn fault remains in place until the inter-turn fault evolves into much larger events that cause enough change in a plurality of signals monitored by the protective relays.
Known stator ground fault detection methods fall into two categories: active and passive methods. The active methods apply a source that generates a low-frequency low-energy square-wave signal. The square-wave signal is injected into the stator of the generator via an appropriately designed coupling circuit. If there is a lack of stator ground fault including a low-resistance connection between the stator and ground, a capacitive current is caused by the square-wave signal. If the stator-ground fault exists, a larger resistive current flows. The active methods are based on detecting a presence or alternatively an absence of the resistive current while using the source to constantly probe the generator. The active methods use extra circuitry including a special interfacing circuitry that needs to be properly isolated, the source, and a current detector. The extra circuitry increases a cost of installation, engineering and maintenance; and calls for extra space to install.
The passive methods monitor signatures in a plurality of available signals and use the difference in these signatures to detect ground faults. A third harmonic neutral voltage is a commonly used passive method. Without a low-resistance path to ground or no stator ground fault, the generator generates a non-zero third harmonic in the so-called neutral point voltage, which is a voltage measured at the neutral point of the generator. With a ground fault, the amount of the third harmonic voltage drops allowing for the detection.
Split-phase protection methods are based on monitoring the split-phase current—a difference between currents in the two parallel windings. This difference signifies a “circulating current” within the parallel windings. In order to provide for the required sensitivity of protection, the difference in currents in measured via window-type current transformer, rather than by paralleling two regular transformers, or other known method. Ideally, with no inter-turn short circuit present such split-phase current should be zero. In practice a non-zero values are measured because of slightly different placement of the two windings within the magnetic circuit of the stator, possible past repairs on the windings (removed coils), asymmetrical coupling with other phases of the machine, etc. Moreover, the amount of the natural split-phase current depends on the operating points of the machine and may change under normal conditions by more than during inter-turn faults. Traditional approach is to monitor the split-phase current and recognize an increased value, or a sudden change in the split-phase current as a sign of inter-turn fault. Finite ability to distinguish natural changes in the split-phase current from changes caused by faults is a major limitation of this traditional approach.